From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id D6487C6FA8F for ; Wed, 30 Aug 2023 08:42:01 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qbGla-0005L8-Nd; Wed, 30 Aug 2023 04:41:34 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qbGlW-0005Kh-RC for qemu-devel@nongnu.org; Wed, 30 Aug 2023 04:41:30 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qbGlT-0005n9-Q7 for qemu-devel@nongnu.org; Wed, 30 Aug 2023 04:41:30 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1693384886; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=iPHlrGm5VCcLhyZ0GMaD8L0jXy3n78/O1shxFNhc6NI=; b=hA1RYyl5xh03PP6NvKZ9k+wETi92I2pmcTSp0xR6jX6fzWt3KZe3IDbwjd3jxpKX1Wa7QG ozhwzvR8p4khDC+MmUt0IA56mlfYQCl6HgDycFwGELACCXnJWJO9GTzt473k3tT6UzOdT3 tFVL39SbAVrvISRUQ6CN7gTLG7mHMd0= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-557-ddmkB2trPQqc6NxCGRc17A-1; Wed, 30 Aug 2023 04:41:25 -0400 X-MC-Unique: ddmkB2trPQqc6NxCGRc17A-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id B28048087B9; Wed, 30 Aug 2023 08:41:24 +0000 (UTC) Received: from [10.39.192.65] (unknown [10.39.192.65]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1BAC8403168; Wed, 30 Aug 2023 08:41:22 +0000 (UTC) Message-ID: Date: Wed, 30 Aug 2023 10:41:21 +0200 MIME-Version: 1.0 Subject: Re: [PATCH 7/7] vhost-user: call VHOST_USER_SET_VRING_ENABLE synchronously Content-Language: en-US To: qemu-devel@nongnu.org Cc: "Michael S. Tsirkin" , Eugenio Perez Martin , German Maglione , Liu Jiang , Sergio Lopez Pascual , Stefano Garzarella , Stefan Hajnoczi References: <20230827182937.146450-1-lersek@redhat.com> <20230827182937.146450-8-lersek@redhat.com> From: Laszlo Ersek In-Reply-To: <20230827182937.146450-8-lersek@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.1 on 10.11.54.10 Received-SPF: pass client-ip=170.10.129.124; envelope-from=lersek@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org I'm adding Stefan to the CC list, and an additional piece of explanation below: On 8/27/23 20:29, Laszlo Ersek wrote: > (1) The virtio-1.0 specification > writes: > >> 3 General Initialization And Device Operation >> 3.1 Device Initialization >> 3.1.1 Driver Requirements: Device Initialization >> >> [...] >> >> 7. Perform device-specific setup, including discovery of virtqueues for >> the device, optional per-bus setup, reading and possibly writing the >> device’s virtio configuration space, and population of virtqueues. >> >> 8. Set the DRIVER_OK status bit. At this point the device is “live”. > > and > >> 4 Virtio Transport Options >> 4.1 Virtio Over PCI Bus >> 4.1.4 Virtio Structure PCI Capabilities >> 4.1.4.3 Common configuration structure layout >> 4.1.4.3.2 Driver Requirements: Common configuration structure layout >> >> [...] >> >> The driver MUST configure the other virtqueue fields before enabling the >> virtqueue with queue_enable. >> >> [...] > > These together mean that the following sub-sequence of steps is valid for > a virtio-1.0 guest driver: > > (1.1) set "queue_enable" for the needed queues as the final part of device > initialization step (7), > > (1.2) set DRIVER_OK in step (8), > > (1.3) immediately start sending virtio requests to the device. > > (2) When vhost-user is enabled, and the VHOST_USER_F_PROTOCOL_FEATURES > special virtio feature is negotiated, then virtio rings start in disabled > state, according to > . > In this case, explicit VHOST_USER_SET_VRING_ENABLE messages are needed for > enabling vrings. > > Therefore setting "queue_enable" from the guest (1.1) is a *control plane* > operation, which travels from the guest through QEMU to the vhost-user > backend, using a unix domain socket. > > Whereas sending a virtio request (1.3) is a *data plane* operation, which > evades QEMU -- it travels from guest to the vhost-user backend via > eventfd. > > This means that steps (1.1) and (1.3) travel through different channels, > and their relative order can be reversed, as perceived by the vhost-user > backend. > > That's exactly what happens when OVMF's virtiofs driver (VirtioFsDxe) runs > against the Rust-language virtiofsd version 1.7.2. (Which uses version > 0.10.1 of the vhost-user-backend crate, and version 0.8.1 of the vhost > crate.) > > Namely, when VirtioFsDxe binds a virtiofs device, it goes through the > device initialization steps (i.e., control plane operations), and > immediately sends a FUSE_INIT request too (i.e., performs a data plane > operation). In the Rust-language virtiofsd, this creates a race between > two components that run *concurrently*, i.e., in different threads or > processes: > > - Control plane, handling vhost-user protocol messages: > > The "VhostUserSlaveReqHandlerMut::set_vring_enable" method > [crates/vhost-user-backend/src/handler.rs] handles > VHOST_USER_SET_VRING_ENABLE messages, and updates each vring's "enabled" > flag according to the message processed. > > - Data plane, handling virtio / FUSE requests: > > The "VringEpollHandler::handle_event" method > [crates/vhost-user-backend/src/event_loop.rs] handles the incoming > virtio / FUSE request, consuming the virtio kick at the same time. If > the vring's "enabled" flag is set, the virtio / FUSE request is > processed genuinely. If the vring's "enabled" flag is clear, then the > virtio / FUSE request is discarded. > > Note that OVMF enables the queue *first*, and sends FUSE_INIT *second*. > However, if the data plane processor in virtiofsd wins the race, then it > sees the FUSE_INIT *before* the control plane processor took notice of > VHOST_USER_SET_VRING_ENABLE and green-lit the queue for the data plane > processor. Therefore the latter drops FUSE_INIT on the floor, and goes > back to waiting for further virtio / FUSE requests with epoll_wait. > Meanwhile OVMF is stuck waiting for the FUSET_INIT response -- a deadlock. I can explain why this issue has not been triggered by / witnessed with the Linux guest driver for virtiofs ("fs/fuse/virtio_fs.c"). That driver registers *two* driver (callback) structures, a virtio driver, and a filesystem driver. (1) The virtio driver half initializes the virtio device, and takes a note of the particular virtio filesystem, remembering its "tag". See virtio_fs_probe() -> virtio_device_ready(), and then virtio_fs_probe() -> virtio_fs_add_instance(). Importantly, at this time, no FUSE_INIT request is sent. (2) The filesystem driver half has a totally independent entry point. The relevant parts (after the driver registration) are: (a) virtio_fs_get_tree() -> virtio_fs_find_instance(), and (b) if the "tag" was found, (b) virtio_fs_get_tree() -> virtio_fs_fill_super() -> fuse_send_init(). Importantly, this occurs when guest userspace (i.e., an interactive user, or a userspace automatism such as systemd) tries to mount a *concrete* virtio filesystem, identified by its tag (such as in "mount -t virtiofs TAG /mount/point"). This means that there is an *abritrarily long* delay between (1) VHOST_USER_SET_VRING_ENABLE (which QEMU sends to virtiofsd while the guest is inside virtio_fs_probe()) and (2) FUSE_INIT (which the guest kernel driver sends to virtiofsd while inside virtio_fs_get_tree()). That huge delay is plenty for masking the race. But the race is there nonetheless. Also note that this race does not exist for vhost-net. For vhost-net, AIUI, such queue operations are handled with ioctl()s, and ioctl()s are synchronous by nature. Cf. : "The original vhost-user specification only demands replies for certain commands. This differs from the vhost protocol implementation where commands are sent over an ioctl() call and block until the back-end has completed." Laszlo > > The deadlock is not deterministic. OVMF hangs infrequently during first > boot. However, OVMF hangs almost certainly during reboots from the UEFI > shell. > > The race can be "reliably masked" by inserting a very small delay -- a > single debug message -- at the top of "VringEpollHandler::handle_event", > i.e., just before the data plane processor checks the "enabled" field of > the vring. That delay suffices for the control plane processor to act upon > VHOST_USER_SET_VRING_ENABLE. > > We can deterministically prevent the race in QEMU, by blocking OVMF inside > step (1.1) -- i.e., in the write to the "queue_enable" register -- until > VHOST_USER_SET_VRING_ENABLE actually *completes*. That way OVMF's VCPU > cannot advance to the FUSE_INIT submission before virtiofsd's control > plane processor takes notice of the queue being enabled. > > Wait for VHOST_USER_SET_VRING_ENABLE completion by: > > - setting the NEED_REPLY flag on VHOST_USER_SET_VRING_ENABLE, and waiting > for the reply, if the VHOST_USER_PROTOCOL_F_REPLY_ACK vhost-user feature > has been negotiated, or > > - performing a separate VHOST_USER_GET_FEATURES *exchange*, which requires > a backend response regardless of VHOST_USER_PROTOCOL_F_REPLY_ACK. > > Cc: "Michael S. Tsirkin" (supporter:vhost) > Cc: Eugenio Perez Martin > Cc: German Maglione > Cc: Liu Jiang > Cc: Sergio Lopez Pascual > Cc: Stefano Garzarella > Signed-off-by: Laszlo Ersek > --- > hw/virtio/vhost-user.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c > index beb4b832245e..01e0ca90c538 100644 > --- a/hw/virtio/vhost-user.c > +++ b/hw/virtio/vhost-user.c > @@ -1235,7 +1235,7 @@ static int vhost_user_set_vring_enable(struct vhost_dev *dev, int enable) > .num = enable, > }; > > - ret = vhost_set_vring(dev, VHOST_USER_SET_VRING_ENABLE, &state, false); > + ret = vhost_set_vring(dev, VHOST_USER_SET_VRING_ENABLE, &state, true); > if (ret < 0) { > /* > * Restoring the previous state is likely infeasible, as well as